Since their discovery, single-walled carbon nanotubes (SWCNTs) have stimulated widespread scientific research due to their promising electronic and mechanical properties. Proven techniques for the production of SWCNTs include carbon arc-discharge evaporation (Iijima et al., “Single-shell carbon nanotubes of 1-nm diameter”, NATURE, Vol. 363, pp. 603-604, Jun. 17, 1993; Ajayan et al., “Growth morphologies during cobalt-catalyzed single-shell carbon nanotube synthesis”, Chem Phys. Lett., 215(5), pp. 509-517, Dec. 10, 1993; Bethune et al., “Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls”, NATURE, vol. 363, pp. 605-606, Jun. 17, 1993), pulsed laser vaporization (PLV) of graphite in the presence of certain metallic catalysts (Thess et al., “Crystalline Ropes of Metallic Carbon Nanotubes”, SCIENCE, vol. 273, pp. 483-487, Jul. 26, 1996), and chemical vapor deposition.
The known methods for producing SWCNTs, however, are unable to produce high quality SWCNT material. Accordingly, the material is refluxed in nitric acid to destroy contaminants that are more easily oxidized than the SWCNTs and then filtered to yield a sheet of tangled nanotubes a fraction of a millimeter thick (i.e., bucky paper). The bucky paper is then vacuum annealed at 1100° C. to drive off C60 molecules and other residual organic impurities.
SWCNTs, produced using a pulsed laser vaporization (PLV) process similar to the one described above, have been investigated using high resolution transmission electron microscopy (HRTEM). Since a carbon nanotube satisfies the weak phase object approximation, its image is a projection of the specimen potential onto a plane that lies normal to the electron beam. The image has maximum contrast where the beam encounters the most carbon atoms, which occurs where the electron beam is tangent to the graphene walls of the carbon nanotube. Indeed, when SWCNTs which are pulled away from the bulk bucky paper were examined using HRTEM, an image consisting of two dark parallel lines separated by about 1.4 nm was obtained. The fact that the space between the parallel lines was clear indicates that no material is present within the SWCNTs. Characterizations of purified PLV-produced nanotubes by X-ray diffraction and Raman spectroscopy confirm these results.
Presently, a SWCNT has the largest elastic modulus (i.e., stiffness) of any intrinsic material. Accordingly, SWCNTs have been used to form reinforced polymer-matrix composites which are useful as high strength, light weight materials. In general, the mechanical properties of the reinforced composites are improved by increasing the stiffness of the reinforcing fibers. As a result, there is a continued need for carbon fibers having even larger elastic modulii.
It has been shown that the stiffness of SWCNTs can be increased by filling it with certain small molecules. Like any other material, when a SWCNT is placed in tension, it undergoes a Poisson contraction that causes a reduction in diameter. By filling-SWCNTs with a molecule having a small compressibility, such as C60, the Poisson contraction is resisted and the elastic modulus is increased. This resistance to transverse deformation also indicates that SWCNTs containing C60 should be less likely to debond from the surrounding matrix, minimizing the likelihood of fiber pull-out, a common failure mode in composites. Use of such filled SWCNTs as reinforcing fibers in polymer-matrix composites should therefore provide a means for improving upon the mechanical properties of polymer-matrix composites.
It has been shown that exterior adsorbates can change metallic SWCNTs into semiconducting SWCNTs, opening the possibility for all-carbon metal-semiconductor junctions at the nanometer length scale. Dopants intercalated in between nanotubes have also been shown to affect the tubes' electronic properties. However, the usefulness of exterior molecules to modify the intrinsic properties of nanotubes is limited by the fact that exterior molecules are accessible to chemical reaction and may be unstable in solvents, vacuum, or certain atmospheres. Interior molecules are hermetically sealed within the nanotube cores, with the nanotube itself forming a steric and kinetic barrier to reaction. It is expected that certain interior molecules, like exterior molecules, will modify the electronic properties of nanotubes. Therefore, it would be highly beneficial to provide a means for permanently modifying the electronic properties of nanotubes by filling to produce novel devices, interconnects, and other technologies.
Mass transport inside a nanotube has been demonstrated by the motion of encapsulated C60 along the axis of the surrounding tube. By the encapsulation of molecules having a strong magnetic moment, one-dimensional, nanoscopic mass transport devices which can be driven by an applied magnetic field are possible. Since even strong magnetic fields are known to be bio-compatible, this has potential application in the targeted delivery of drug molecules by the field induced eject ion of the molecules from the nanotube cavity into the target (i.e., a nano-syringe). Mass transport may also be driven by other means (e.g., by momentum transfer from a laser). In all cases, the nanotube can be utilized to direct the motion of molecules in a controlled manner and along specific pathways.
Nanotubes also show promise for use as chambers to produce or catalyze the production of molecular structures that could not be easily produced otherwise. For example, a nanotube can act to enhance a reaction rate by confining reactants in close proximity, and it can determine the reaction product by providing a steric constraint on its structural form. Despite its small diameter, a nanotube is sufficiently strong to be self supporting over distances that are large as compared to many molecules. The nanotube therefore provides a convenient means to stabilize individual molecules for high signal-to-noise ratio characterization, such as by electron beam techniques, without the interference from a substrate film.
In light of the foregoing, SWCNTs and other types of single-walled nanotubes (SWNTs) show promise as reinforcing fibers for polymer-matrix composites, novel electronic devices, and nanoscopic mass transport devices (including nanoscopic drug delivery systems). The use of nanotubes in such applications and others is dependent upon the ability to controllably modify the intrinsic (e.g., mechanical, electronic, and/or magnetic) properties of the SWNTs by manipulating their microstructure. A particularly promising means for altering the intrinsic properties of SWNTs and using SWNTs in novel ways involves the filling of nanotube cavities or lumens to produce hybrid molecular assemblies that can have novel functionality. Accordingly, there is a need for nanotubes containing small molecules within their interior lumens and methods for producing the same.